Iranica Journal of Energy & Environment 5 (1): 65-69, 2014 ISSN 2079-2115 IJEE an Official Peer Reviewed Journal of Babol Noshirvani University of Technology DOI: 10.5829/idosi.ijee.2014.5.1.5110 BUT
Simulation and Optimization of LNG Production Unit for Energy Conservations Ali Tarjoman Nejad and Ali Farzi Department of Chemical Engineering, University of Tabriz, P.O. Box: 5166616471, Tabriz, Iran (Received: January 6, 2014; Accepted in Revised Form: March14, 2014)
Abstract: The prospect of LNG could become a major global energy source is one of the most debated issues. The Liquefied Natural Gas (LNG) supply chain and the properties that make this fuel environmental friendly is in high demand for energy supply. In this paper, at first, the process of converting the natural gas to LNG was simulated; then, the process is optimized to archive minimum energy consumption per ton of LNG produced. Using a three stage exchanger is the best way for minimization of energy consumption in LNG production unit. Outlet pressure from the compressor and also type of refrigerant in cooling system is very effective on rate of energy conservations. The best mass fraction for refrigerants in liquefaction cycle are 0.88 for methane and 0.12 for ethane. For subcooling cycle that fraction is defined as 0.6 for methane and 0.4 for nitrogen. The optimized pressure in outlet of compressors in liquefaction cycle is 650 kPa; also, for the subcooling cycle is 1800 kPa. The amount of consumed energy was 14.91 kW per ton of produced LNG. Key words: Natural gas
LNG
Energy optimization
INTRODUCTION
Simulation
Refrigeration cycle
Process Description: LNG exchanger should be used for cooling natural gas temperature to -161 °C. Three stage exchangers can be used for this purpose. In first stage which is precooling stage, natural gas is cooled down to -40°C. In the second stage which is liquefaction stage, natural gas is cooled to -90 °C. Finally, in the third stage which is subcooling stage, natural gas is cooled down to -161 °C [4]. The plate-fin heat exchanger are often used to achieve such temperature. A brazed aluminium plate-fin heat exchanger consists of a block of alternating layers of corrugated fins. The layers are separated from each other by parting sheets and sealed along the edges by means of side bars and are provided with inlet and outlet ports for the streams. The block is bounded by cap sheets at top and bottom. The minimum design temperature for this heat exchanger is -269°C [4]. In general, fluids should be clean, dry and noncorrosive to aluminium. Trace impurities of H 2S, NH 3, CO2, SO2, NO2, CO, Cl2 and other acid-forming gases do
For production of LNG the natural gas is cooled down to temperature of -161°C. In this case, the gas turned to an odorless and transparent liquid with density about 450 kg/m3. The gas volume is reduced to 1/600 with liquefaction which economizes transportation of natural gas [1]. LNG is an environmental friendly fuel and this factor together with economic factors causes this fuel to be very important and be pursued [1]. LNG market in Asia, Northern America and Europe progressively developed in past decades [2]. For production of LNG, the natural gas temperature should decrease to -161 °C while the pressure has to increase to 5 bar. Various refrigerants can be used for LNG process. The best and the most available ones are: R-22, methane, ethane and nitrogen [3]. In this work, these refrigerants have been used for the liquefaction of natural gas.
Corresponding Author: Ali Tarjoman Nejad, Department of chemical Engineering, University of Tabriz, P.O. Box: 5166616471, Tabriz, Iran. Tel: +98-411-3393158, Cell: +98-937-7786511, E-mail:
[email protected].
65
Iranica J. Energy & Environ., 5 (1): 65-69, 2014
In second stage, the first stream enters to the compressor for increasing its pressure, then it enters into a chiller for the reduction of its temperature. At last, it enters into a turbine and then enters LNG exchanger. Subcooling: In the third stage, the stream enters a compressor for increasing its pressure, then it enters the first LNG exchanger of its precooling. Then, it enters a turbine and its pressure is reduced and finally it enters the second LNG exchanger. In this stage, the temperature of refrigerants stream should be -165 °C in order to reduce the temperature of natural gas to -161 °C. Boil-off Gas (BOG) is the vapor phase in LNG tanks. The increase in BOG will lead to an increase in pressure of LNG tank, as the specific volume of gas is much larger than that of liquid form. It seems that BOG can be a serious problem for LNG storage tanks [9]. At the end of the process, LNG enters the storage tanks where 5% of LNG is evaporated. Boil-off gas is compressed to the pressure of natural gas and is recycled and added to the feed.
Fig. 1: The actual process flow diagram not create corrosion problem in streams with water dew point temperatures lower than the cold-end temperature of the brazed aluminium plate-fin heat exchanger [5]. Acid gas removal is performed first to reduce carbon dioxide levels to around 50 ppm. To remove CO2 from natural gas, amine tower can be used [6]. The process flow diagram of LNG production is shown in Fig. 1. Precooling: In precooling stage, R-22 has been used as a refrigerant and J-T valves are also used. The J-T valve can be designed for any pressure drop.Joule-Thomson effect is made by using j-t valve. When Joule-Thomson coefficient is positive, a drop in pressure means a drop intemperature. In this situation Joule-Thomson coefficient for R-22 will be positive. So, Temperature in the cycle will decrease. Inlet natural gas feed is in temperature of 25 °C and the pressure of 7 bar. In inital stage, the temperature is reduced to -40 °C. The temperature cannot be reduced to lower than the stated temperature using R-22. In this stage, a flash drum is used for the separation of LPG gas. In flash drum, water is separated in addition to LPG. In order to prevent freezing of water in the subsequent stages, separation of water is very important in the cryogenic process. It is curcial for water vapor content compressed gas should be reduced to lower than 1ppmv [7].
Simulation: ASPEN HYSYS software was used for the simulation of LNG production unit. Because, in this simulation available components are hydrocarbon and nonpolar, Peng Robinson equation of state is used for thermodynamic calculations [10]. The simulation environment for the process is shown in Fig. 2. The properties of produced LNG are summarized in Table 1. Optimization: The effective parameters on energy consumption should be recognized for the optimization of consumed energy. The kind of refrigerant important in consumed energy. For the stages it is more desired to use the refrigerants in such a way that temperature ranges of the stages have optimal performance. It is suitable to use more than one refrigerant in the stage to increase the range of existence two phases. Two refrigerants of methane and ethane are appropriate for liquefaction stage while two refrigerants of methane and nitrogen are appropriate for subcooling stage. This choice is due to high efficiency of these refrigerants within the temperature ranges of stages. The best mass fraction of refrigerants should be specified for both cycles. The consumed energy per ton of LNG versus the mass fraction of the refrigerants for liquefaction and subcooling cycles are shown in Figs. 3 and 4.
Liquefaction: In this stage, one LNG exchanger with three streams is used. Although one refrigerant can be used for cooling, but use of mixture of two refrigerants is more desired, because it guarantees more ranges for existence of two phases [8]. The first stream is a mixture of methane and ethane. This stream is a part of stage which includes a compressor, a chiller and a turbine. The second stream is a mixture of nitrogen and methane. It is cooled in the LNG exchanger for using in third exchanger i.e. subcooling stage. 66
Iranica J. Energy & Environ., 5 (1): 65-69, 2014
Fig. 2: LNG unit simulation enviroment Table 1: The properties of prouduced LNG obtained by simulation Parameters
Value
vapor phase fraction
0
Temprature (°C)
-160
Pressure (kPa)
500
mass density (kg/m3)
458.357
mass heat capacity (kJ/kg.C)
3.192
Viscocity (cP)
0.113
Thermal conductivity (W/m.k)
0.186
Mass heat of vaporization (kCal/kg)
156.047
The results showed that the best mass fraction of refrigerants in liquefaction stage defined as 0.88 for methane and 0.12 for ethane. In subcooling stage, consumed energy decreases with decreasing of nitrogen mole fraction, but LNG exchanger can’t act properly for mass fractions lower than 0.4 and temperature cross is occurred in the exchanger. Therefore, the mass fraction of nitrogen was considered to be 0.4. In Table 2 shows the mass fraction of refrigerants for liquefaction and subcooling stages. The outlet pressure of compressor has very important effect on energy consumption. When the pressure increases, the turbine outlet temperature can be reduced, but this leads to an increase in energy consumption of compressor [11]. When the pressure is reduced, more energy in chiller is required. Therefore, the optimized pressure should be considered for the stages. In Fig. 5 depicts the consumed energy per ton of
ig. 3: Consumed energy per ton of produced LNG vs the mass fraction of Methane in liquefaction cycle
Fig. 4: Consumed energy per ton of produced LNG vs the mass fraction of Nitrogen in sub-cooling cycle 67
Iranica J. Energy & Environ., 5 (1): 65-69, 2014
Fig. 8: PVT diagram for subcooling cycle Fig. 5: Consumed energy vs the outlet pressure from compressor for liquefaction cycle
Fig. 9: PVT diagram for final LNG product Table 2: Optimal refrigerants mass fraction obtaind by ASPEN HYSYS Stage Refrigerant Mass fraction liquefaction Ethane 0.12 Methane 0.88 Subcooling Nitrogen 0.4 Methane 0.6
Fig. 6: PVT diagram for precooling cycle
Table 3: Specifications of streams in liquefaction stage Variables E-M1 E-M2 E-M3 Vapor fraction 0.963 1 1 Temperature (°C) -115.1 -112.7 -78.2 Pressure (kPa) 400 390 700 Table 4: Specifications of streams in subcooling stage Variables 1 2 3 4 Vapor fraction 0.345 0.999 1 1 Temperature (°C) -165 -150.4 -71.8 -60 Pressure (kPa) 400 390 1800 1790
Fig. 7: PVT diagram for liquefaction cycle
E-M4 1 -103.3 700
5 1 -74 1780
6 1 -126.8 420
pressure for prevention of temperature crosses in LNG exchanger and reach to the desired temperature is 1800 kPa. If the pressure increases to high value, then the consumed energy will also increase. The temperature and pressure of each of the streams in liquefaction and subcooling stages are summarized in Tables 3 and 4. At optimal conditions, amount of consumed energy per ton of LNG is 14.91 kW.
produced LNG versus the outlet pressure of the compressor for liquefaction stage. The optimized pressure for this compressor is 650 kPa. In subcooling stage, the compressor outlet pressure should be high, because of the refrigerants are needed in very low temperature. In this situation, the least possible 68
Iranica J. Energy & Environ., 5 (1): 65-69, 2014 Table 5: Comparison between various designs Design Conoco Phillips Prico Dual TEX Cycle Kryopak EXP This work
Process
Compression Eff. %
Optimized Cascade Refrigeration Single Mixed Refrigeration Turbo-Expander Turbo-Expander Three stage exchanger - Mixed Refrigeration
100 100 100 100 75
kW/ton 14.1 16.8 16.5 15.5 14.91
REFERENCES
In Figs. 6 to 8, PVT diagram for precooling, liquefaction and subcooling cycles are shown. According to the presented data, with the increasing number of materials in stream, two-phase region is broader. Operating Temperature cycles from the first to the third cycle is reduced. In Fig. 9, PVT diagram for final LNG product is shown. The final product is in liquid zone. In this paper simulated design is compared with some conventional processes. This comparison is shown in Table 5. Only ConocoPhillips method has less energy consumption, but the energy consumption for this process is calculated in 100% compression efficient while in this design the efficiency is intended to 75%. So the design has lower energy consumption in real situation.
1.
CONCLUSIONS Using a three stage exchanger is the best arrangements for the optimization of energy consumption in LNG production unit. The outlet pressure from the compressor and the kind of refrigerants are very effective on energy consumption. Energy consumption can be reduced by obtaining the optimized pressure for the compressor and by obtaining the best mass fractioin for the refrigerants. The best mass fraction of refrigerants in the liquefaction stage is 0.88 for methane and 0.12 for ethane and for subcooling stage is 0.6 for methane and 0.4 for nitrogen. The optimal outlet pressure from the compressor in liquefaction and subcooling stages were 650 and 1800 kPa, respectively. In this optimal condition, consumed energy per ton of LNG is 14.91 kW.
69
Aoki, I. and Y. Kikkawa, 1995. Technical efforts focus on cutting LNG plant costs. Oil and Gas Journal, 93(27). 2. Kikkawa, Y., M. Nakamura and S. Sugiyama, 1997. Development of liquefaction process for natural gas. Journal of Chemical Engineering of Japan, 30(4): 625-630. 3. Geist, J., 1985. Refrigeration cycles for future baseload LNG plants need a close look.Oil and Gas Journal, 83(5). 4. Habibullah, A., P. Lardi and M. Passmore, 2009. LNG conceptual design strategies. in The 88th GPA Annual Convention in San Antonio, TX. 2009. 5. ALPEMA, 2000. The standard of the brazed aluminium plate-fin heat exchanger manufacturers association. Second Edition. 6. Lorn, W. Liquefied Natural Gas, 1974, Applied Science Publishers Ltd, Barking. 7. Jamieson, D., P. Johnson and P. Redding, Targeting and achieving lower cost liquefaction plants. in Twelfth International Conference and Exhibition on Liquefied Natural Gas, Perth, Australia. 1998. 8. Haywood, R., 1990. Analysis of Engineering Cycles, 1990, Pergamon Press, Oxford. 9. Aimikhe, V., B. Kinigoma and E. Iyagba, 2013. Predicting the Produced Boil off Gas in the Moss Spherical Liquefied Natural Gas (LNG) Vessel. in SPE Nigeria Annual International Conference and Exhibition. 2013. Society of Petroleum Engineers. 10. Carlson, E.C., 1996. Don't gamble with physical properties for simulations. Chemical Engineering Progress, 92(10): 35-46. 11. Alabdulkarem, A., A. Mortazavi, Y. Hwang, R. Radermacher and P. Rogers, 2011. Optimization of propane pre-cooled mixed refrigerant LNG plant. Applied Thermal Engineering, 31(6): 1091-1098.
